Study On The Dynamic Evolution Processes Of Hydrate Formation And Decomposition Based On ERT Technique

There are many kinds of distribution forms of natural gas hydrate in the natural formation,such as layer,block and concretion.As a non-destructive visual measurement technique,Electrical Resistance Tomography can obtain the conductivity image inside the measured medium,which is more conducive to understand and study the electrical response characteristics and hydrate accumulation and decomposition mechanism under the heterogeneous distribution.By applying ERT to the hydrate electrical study test and establishing the hydrate test method based on ERT technique,this research provides test technical means and data support for the study on the dynamic mechanism of the formation and decomposition process of natural gas hydrate.It mainly includes the following contents:Independently designed and built the hydrate ERT experimental test device,and tested the process of ice formation and melting in sediments.The test results show that the formation factors obtained at different temperatures are basically stable,indicating that the resistivity measurement results of the device are reliable.According to the measured conductivity distribution results,all stages of the ice formation and melting process can be identified,which verifies the feasibility of the application of ERT technology in hydrate experimental test,laying a foundation for the research on the formation,decomposition and evolution process of hydrate based on ERT technology.The formation and decomposition process of pore-filled and fracture hydrate are tested and analyzed with ERT test device,and the experimental test method and data processing method suitable for hydrate are established,including experimental test flow,preparation method of samples and image processing method.The average resistivity characteristics and the conductivity spatial distribution characteristics during the process of hydrate formation and decomposition are analyzed,the results show that:(1)By establishing the experimental test method and data processing method for hydrate,the electrical characteristics and the hydrate spatial distribution during the formation and decomposition of hydrate can be studied effectively;(2)During the hydrate formation process,when the saturation is small,the average resistivity increases slightly and when the saturation is large,the increase is larger,which is affected by multiple factors such as salt discharge effect and hydrate saturation;(3)During the hydrate decomposition process,there is a phase front of hydrate decomposition,the average resistivity firstly decreases rapidly and then rises rapidly.The more the hydrate decomposition amount,the greater conductivity increase and decrease,and the abnormal decrease of conductivity of the analysis region is caused by the increase of gas phase saturation.(4)For the fracture-type hydrate,the rate of increase of conductivity in the inner end region of the fracture along the radial direction of the core is higher and the final increase is larger,indicating that the hydrate was generated preferentially at this position and the final amount is more.Firstly,the ERT simulation model based on Matlab is established.Then the reliability of the ERT positive problem model is verified by experimental data.Finally,the positive problem simulation and image reconstruction and evaluation are carried out for the typical crack distribution.Comparing the electrical conductivity distribution setting images and reconstructed images of three typical fracture-type hydrates,the results show that the established ERT simulation model can better distinguish the shape and size of fracture-type hydrates.The ERT simulation model can be used to simulate the conductivity response characteristics of hydrates under more complex fracture conditions,which is conducive to expanding the scope of simulation experiments,thus laying the data foundation for the effective application of ERT technology in hydrate electrical parameter testing.